RU2410222C2 - Replanishable leadless solder and method of concentrating copper and nickel in soldering bath - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: part, a p.c.b. coated by copper film, or a copper wire terminal, or a copper strip terminal, is dipped into, soldering bath, said part after soldering being subjected to processing by air knife or stamp. After Cu concentration in bath with solder exceeds check value and Ni concentration drops below said value, replenishing leadless solution is fed into bath, consisting mainly of Sn and containing, at least, Ni in amount of 0.01 wt % to 0.5 wt %. After feeding leadless solder of said composition, solder concentration, changed abruptly, quickly returns into required range.

EFFECT: soldering without changing solder.

5 cl, 2 dwg

Description

FIELD OF THE INVENTION

The present invention relates to lead-free solder, which is replenished with the aim of regulating the concentration of copper (Cu) and nickel (Ni) in the bath for brazing by immersion at a rapid change in their concentrations under the influence of specific conditions of the subsequent process, as well as to a method for controlling the concentration of Cu and Ni in said bath under specific conditions.

BACKGROUND OF THE INVENTION

Known quasi-eutectic tin-lead (Sn-Pb) solder, which is widely used due to its low melting point and reliability. Nevertheless, taking into account environmental requirements, the need for lead-free solder is growing. Currently, lead-free tin-copper (Sn-Cu) solder is widely used, in particular tin-copper-nickel (Sn-Cu-Ni) solder with a higher fluidity than other lead-free solders. Sn-Cu-Ni solder is preferred because it eliminates soldering defects, including smoothing the surface of the solder, jumpers, connections with a through hole, unsoldered joints, etc., which can cause problems in mass production.

The printed circuit board of electronic devices coated with a copper film and the components of the electronic circuit with copper wire or ribbon leads are immersed in a soldering bath. Copper is able to dissolve in an immersion soldering bath, as a result of which its concentration in the bath gradually rises. As a result, an Sn-Cu intermetallic compound with a high melting point is formed that does not melt at a given melting point in the bath. This connection engages with the solder part. Thus, the soldering quality is degraded. Japanese Open Gazette, No. 2001-237536, proposes a concentration control technique to overcome this drawback. According to the proposed method, an additional solder with a low concentration of Cu is added to the bath in order to maintain the concentration of Cu at a constant level or below it.

Disclosure of invention

Objectives of the invention

After tinning, by immersing the part in the soldering bath and then removing it from the soldering bath, alignment is usually carried out using the so-called solder equalizer with an air knife (hot air), hereinafter referred to as HASL (from the English - hot-air solder leveler). During the alignment process, excess solder is removed with an air knife by blowing the part with high temperature and pressure air. As a result of this operation, the increase in concentration significantly exceeds the expected change in the concentration of Cu in the dip bath according to the described method for controlling the concentration of Cu. More precisely, despite the small processing area of the part, the rate of increase in the concentration of Cu in the bath is very high. This particular phenomenon also occurs when drawing parts using a stamp.

When HASL removes excess solder from a part after it is immersed in a solder bath, the Ni concentration in Sn-Cu-Ni lead-free solder in the bath drops rapidly, despite the small workpiece area. A decrease in the concentration of Ni leads to a deterioration in the fluidity of the molten solder and the formation of defects to smooth the surface of the solder, holes, unsoldered joints and other defects. In the context of mass production, these defects can create serious difficulties, such as stopping an entire production line. In this regard, the regulation of the concentrations of Cu and Ni in the bath for brazing by immersion is a very important factor in ensuring the reliability of production.

The elimination of these disadvantages is the basis of the present invention, the task of which is to create a lead-free solder, replenished with the aim of regulating rapidly changing concentrations of Cu and Ni and bringing them to an acceptable range without replacing the solder in the bath for brazing by immersion, and also a method of controlling the concentration of Cu and Ni.

Means for solving the problems of the invention

The author of the present invention conducted research to obtain a solution that responds to a rapid change in the concentration of Cu and Ni in the molten solder in the bath for brazing by immersion, which was not feasible by conventional methods. More precisely, the specific conditions for the use of HASL or a stamp in a subsequent process have been investigated. Specific conditions differ from the conditions of the concentration control method adopted by wave soldering, when the molten solder is sprayed and a wave is created, and Cu is dissolved only when the wave collides with the back surface of the printed circuit board.

For example, since HASL atomizes high temperature and pressure air, the atomized air exerts a certain impact on the part itself. As a result, not only the solder is removed, but also a certain amount of Cu located on the surface of the part. In the course of further studies, it was found that the physical force (impact, etc.) by which HASL and the stamp act on the part removes the solder layers, including the layer at the (Ni, Cu) ₆ Sn ₅ interface formed between Cu on surface of the part and a layer of solder.

Cu, which is more present on the surface of the part than on the solder layer, easily moves into the layer at the interface between the part and the solder layer, resulting in a layer with a high content of Cu. If the solder layer, including the layer at the interface between the solder and the part, is removed using HASL or a similar device, the concentration of Cu rapidly increases as a result of the return of the solder to the soldering bath by immersion in it.

As a result of the rapid increase in the concentration of Cu in the dip bath, an Sn-Cu intermetallic compound is formed.

The Sn-Cu intermetallic compound remains at the bottom of the bath. Since it absorbs Ni from the bath during crystallization of the Sn-Cu intermetallic compound, the Ni concentration in the bath drops rapidly. Even if the operating temperature in the bath is maintained at about 260 ° C. in order to slow down the formation of the Sn-Cu intermetallic compound, a rapid increase in the Cu concentration cannot be controlled. As a result, an Sn-Cu intermetallic compound forms at the bottom of the bath, which naturally leads to the capture of Ni. Ni, which is present in small amounts in lead-free solder, is usually not found in parts such as printed circuit boards. With every part treatment, the Ni concentration in the bath decreases. Since a small amount of Ni is a significant factor in Sn-Cu-Ni solder providing high fluidity, it is important to replenish Ni in a timely manner to ensure production reliability, the amount of which decreases as a result of processing the part.

In the case of a specific device, such as HASL, a sharp change in the concentration of Cu and Ni in the bath for soldering by immersion, in connection with which it is necessary to accurately control the concentration. As a result of in-depth studies of the above problems, it became clear the need to additionally replenish the amount of solder to increase the Cu concentration by a predetermined amount, set as the maximum concentration change, and to reduce the Ni concentration by a predetermined amount, set as the maximum concentration change. It was also found that optimal regulation of the concentration cannot be achieved without regulating the composition of the replenished lead-free solder.

The lead-free solder proposed in the present invention is replenished in a bath in which a component consisting of a printed circuit board coated with a copper film, a copper wire terminal or a copper tape terminal is placed for subsequent soldering using an air knife or die. Lead-free solder contains Sn as the main component and at least Ni, the concentration of which is in the range from 0.01 wt.% Inclusive to 0.5 wt.% Inclusive. As a result of replenishment of lead-free solder with a given composition in the bath, the concentration of which changed due to the use of HASL or a stamp during subsequent processing, its composition is quickly restored to the corresponding concentration range.

The maximum permissible concentration of Cu in a lead-free solder, in which the concentration of Cu and the concentration of Ni in the dipping brazing bath is controlled, is 1.2 wt.%. Preferably, the concentration of Cu is 0.7 wt.% Or less, based on the regulation of the formation of an intermetallic compound, such as an excess of (Ni, Cu) ₆ Sn ₅ .

The eutectic point Sn-Cu corresponds to 0.7 wt.%. More preferably, the concentration of Cu is 0.5 wt.% Or less. Most preferred is a lead-free Sn-Ni solder that does not contain Cu at all, since the concentration of Cu in the entire bath is most rapidly reduced. On the other hand, the concentration of Ni is preferably in the range from 0.05 wt.% Inclusive to 0.3 wt.% Inclusive. The eutectic point of Sn-Ni corresponds to 0.15 wt.%, And a deviation of 0.1 wt.% To the lower side and 0.15 wt.% To the larger side from 0.15 wt.% Is set as the interval. It is assumed that in this interval, it is easy to adjust the replenishment of Ni. The most preferred range is from 0.1 wt.% Inclusive to 0.2 wt.% Inclusive. In this interval, the change in the concentration of Cu during replenishment of the solder is regulated in a narrow range. Thus, stabilize the composition of the solder.

The inventive method for controlling the concentrations of Cu and Ni in solder in a bath includes a stage in which a part consisting of a printed circuit board coated with a copper film, a copper wire lead or a copper tape output is immersed in the bath to solder and the solder removed is returned with a part with an air knife or a stamp into the bath. Lead-free solder is replenished in the bath before the concentration of Cu in the bath rises by a maximum of 0.5 wt.% Relative to its predetermined value, and the Ni concentration in the bath decreases by a maximum of 0.03 wt.% Relative to its predetermined value, while the replenished lead-free the solder contains Sn as the main component, Cu in a concentration of 1.2 wt.% or less, and Ni in a concentration ranging from 0.01 wt.% inclusive to 0.5 wt.% inclusive. According to this method, rapidly changing concentrations in the bath are measured continuously or continuously, and the solder is replenished before a predetermined increase in value or a predetermined decrease in concentration is achieved. Appropriate concentration control and a decrease in the frequency of soldering defects occur even in the case of a rapid change in the concentrations of Cu and Ni in the bath as a result of the use of HASL or a die during subsequent processing. This ensures reliable production without interrupting the soldering process.

According to the control method proposed in the present invention, the lead-free solder is replenished before the Cu concentration increases by a maximum of 0.3 wt.% Relative to its predetermined value, and the Ni concentration decreases by a maximum of 0.02 wt.% Relative to its predetermined value. Due to this, a sufficient margin is maintained to ensure the reliability of production, and a low operating temperature is maintained in the dip bath. These indicators are preferred in terms of production reliability. Regarding the composition of the refillable lead-free solder, the above preferred ranges of Cu and Ni concentrations are applicable in the control method proposed in the present invention.

The advantages of the present invention are provided by any of the aforementioned embodiments if the lead-free solder for replenishment in addition to Cu and Ni contains germanium (Ge) and phosphorus (P) as an oxidation inhibitor at a concentration of about 0.1 wt.% Each. The addition of Ge is more likely to prevent the leaching of Cu than the addition of P. Even a lead-free solder that does not contain elements other than Sn, Cu and Ni is able to provide the advantages of the present invention. If we compare the lead-free solder, additionally containing Ge and P, and the lead-free solder, containing only Sn, Cu and Ni without additional elements, the first of them is preferable. Lead-free solder, additionally containing Ge and P, limits the oxidation of the solder, due to which less oxides are formed (less scale) and fewer oxides are adhered to products. Refillable lead-free solder containing Co instead of Ni has the same drawback as solder containing Ni, but is overcome in the present invention. Any form, for example, wire rods, wires, etc., may be added to the replenishment solder, while any form of solder provides the advantages of the present invention.

Result of invention

When the part is removed from the bath after being immersed and subjected to HASL or die processing, the replenishing solder of the present invention immediately restores the Cu concentration and Ni concentration rapidly changing during processing to the appropriate concentration range. According to the present invention, replenishing solder prevents the formation of various defects, such as holes and unsoldered joints, as a result of poor fluidity of molten lead-free solder Sn-Cu-Ni. Reliable manufacturing of the part is ensured. According to the method of controlling the Cu concentration and Ni concentration in the bath proposed in the present invention, the frequency of occurrence of defects as a result of the corresponding regulation decreases even with a rapid change in the concentration of Cu and Ni in the bath. A reliable production process, in particular mass production, is continued without interruption of the soldering process.

Brief Description of the Drawings

Below the invention is described in more detail with reference to the accompanying drawings, which show:

Figure 1 - graph of changes in the concentration of Cu and the concentration of Ni in the bath for soldering by immersion in the process of continuous soldering using an air knife (scraper) in the process following soldering,

figure 2 is a graph of changes in the concentration of Cu and the concentration of Ni in the bath during continuous brazing using an air knife in the following brazing process in accordance with the present invention.

Preferred Embodiments

Embodiments of the present invention are described below. An embodiment is first described in which an HASL device air knife is used to remove excess solder from a printed circuit board after it has been immersed in a soldering bath.

In this embodiment, a lead free solder is used that does not contain any elements other than Sn, Cu and Ni. More precisely, the lead-free solder contains 0.7 wt.% Cu, 0.05 wt.% Ni and Sn, which accounts for the remaining percentage concentration. Lead-free solder melts at a temperature of 265 ° C in a dip bath. Under these conditions, the part, which is the printed circuit board, is lowered vertically down and left immersed in molten solder in the bath for 1 to 5 seconds, and then removed from the bath at a speed of 10 cm / s to 20 cm / s. Using a HASL air knife, air heated to a temperature of 280 ° C is sprayed from both positions of the opposite sides of the printed circuit board from two positions, as a result of which, when the part is removed, it is affected by air pressure from 0.098 MPa to 0.294 MPa. This soldering operation is repeated several times in a row and the Cu concentration and Ni concentration in the bath are measured at each treatment of the part.

Figure 1 shows a graph of the change in the concentration of Cu and the concentration of Ni in the bath during the sequential implementation of the soldering process described above. In this case, the ordinate (Y axis) shows the concentration of Cu (wt.%) And the concentration of Ni (wt.%), And the abscissa (X axis) shows the work area (m ² ) of the part, which is the printed circuit board. As shown in FIG. 1, the concentration of Cu in the bath rises rapidly, despite the small processing area of the part, and the concentration of Ni decreases rapidly with increasing processing area of the part. With continued brazing and processing with HASL, insoluble Sn-Cu intermetallic compounds begin to form, even if the operating temperature is maintained at 260 ° C. or higher. The concentration of Cu in the bath stops increasing when a large number of Sn-Cu intermetallic compounds form on the walls and bottom of the bath. Because of this, it is necessary to stop the soldering operation. The concentration of Ni continues to decrease even after the concentration of Cu ceases to increase. This is because Sn-Cu intermetallic compounds increase Ni uptake. It was found that the sharp initial change in concentration is approximately 10 times higher than the rate of change in the concentration of Cu during wave soldering (in other words, the concentration gradient of Cu increases when processing a printed circuit board of a standard area).

The author received a replenishing lead-free solder containing 0.15 wt.% Ni and Sn, which accounts for the remaining percentage concentration. As in the case described above, the soldering operation is carried out on a printed circuit board. Replenishing solder is introduced into the bath and the Cu concentration and Ni concentration in the bath are measured. More precisely, the replenishing lead-free solder described above is introduced into the bath before the Cu concentration by 0.5 wt.% Exceeds the control value and the Ni concentration decreases by 0.03 wt.% Relative to the control value. Figure 2 shows a graph of the change in the concentration of Cu and the concentration of Ni in the bath for soldering by immersion. The arrows indicate the time points at which the replenishing lead-free solder was introduced. Before the soldering operation, control values indicate the initial values of the Cu concentration and Ni concentration in the bath, and during the soldering operation, the values of Cu concentration and Ni concentration after the maximum change in the Cu concentration and Ni concentration as a result of introducing replenishing lead-free solder (more precisely, the maximum value for Cu and the minimum value for Ni).

As shown in FIG. 2, with an increase in the machined area of the part, the Cu concentration rapidly increases, and the Ni concentration rapidly decreases. With the introduction of the replenishing lead-free solder described above, the Cu concentration and Ni concentration are quickly restored to values close to their original values. As shown, the replenishing lead-free solder described above is introduced several times, and an increase in the concentration of Cu and a decrease in the concentration of Ni are monitored with a concentration meter. The concentration of Cu and the concentration of Ni are controlled to within appropriate intervals with good reproducibility. The amount of lead-in replenishment added to the lead-free solder is appropriately determined based on the ratio of the amount of molten solder in the bath. As for the corresponding ranges of the concentration of Cu and the concentration of Ni, the concentration of Cu is from 0.6 wt.% Inclusive to 1 wt.% Inclusive, and the concentration of Ni is from 0.02 wt.% Inclusive to 0.08 wt.% Inclusive.

In another embodiment of the same process and the above measurements, lead-free solder is added containing Sn as the main component, Cu at a concentration of more than 1.2 wt.% And Ni at a concentration of less than 0.01 wt.% Or more than 0.5 wt. % In this case, even with the introduction of lead-free solder at any time during the soldering operation, the increase in the concentration of Cu in the bath for immersion soldering cannot be controlled, and the concentration regulation does not bring any result. At the same time, a lead-free solder containing Cu in a concentration of less than 1.2 wt.% And Ni in a concentration ranging from 0.01 wt.% Inclusive to 0.5 wt.% Inclusive, allows you to adjust the concentration. In particular, the regulation of the concentration is relatively quickly carried out using a lead-free solder in which the concentration of Cu is equal to or less than 0.7 wt.%. When the concentration of Cu is equal to or less than 0.5 wt.%, The regulation of the concentration is carried out even faster. The fastest and most reliable way to control the concentration of Cu is when the lead-free solder does not contain Cu at all.

In another embodiment, in a lead-free solder containing Sn as the main component, Cu in a concentration equal to or less than 1.2 wt.%, Ni in a concentration ranging from 0.01 wt.% Inclusive to 0.5 wt.% Inclusive stabilize the concentration of Ni in the bath accurate to the appropriate interval, in particular from 0.02 wt.% inclusive to 0.08 wt.% inclusive. If the Ni concentration is in the range from 0.05 wt.% Inclusive to 0.3 wt.% Inclusive, when adjusting the concentration of Ni in the bath, it is more quickly adjusted with accuracy to the appropriate interval. The decrease in the concentration of Ni in the bath is most quickly restored if the concentration of Ni is in the range from 0.1 wt.% Inclusive to 0.2 wt.% Inclusive. At the upper limit concentration of Ni, the formation of Sn-Cu intermetallic compounds is reliably limited.

If a HASL or die is used in the present and another embodiment for the subsequent brazing treatment, the operating temperature in the immersion brazing bath is set to be from 260 ° C inclusive to 300 ° C inclusive. Due to this, the formation of refractory Sn-Cu intermetallic compounds in the bath is regulated as a result of a sharp change in the concentration of Cu and Ni under the action of special conditions following the melting process. Due to this, provide a sufficient margin of time for the introduction of replenishing lead-free solder having each of the above compositions. In other words, the lead-free solder introduced into the bath can have a wide composition. Taking into account the mentioned adjustment of the operating temperature, even a lead-free solder containing Cu in a concentration equal to or less than 1.2 wt.% Is also able to provide concentration control in the bath. The upper limit temperature is 300 ° C, since at a temperature above 300 ° C in the bath there is an excessive dissolution of Cu located on the part, and even when using replenishing lead-free solder, concentration control becomes extremely difficult. Based on this, the most preferred upper limit of the operating temperature in the bath is 280 ° C.

In another embodiment, the lead-free solder is introduced before the Cu concentration has increased by a maximum of 0.3 wt.% Relative to its predetermined value, and the Ni concentration has decreased by a maximum of 0.02 wt.% Relative to its predetermined value. The time of continuous operation is extended in comparison with the use of lead-free solder, which is introduced with a more significant change than discussed above. The operating temperature in the bath is in a relatively lower range from 262 ° C to 263 ° C.

The following describes another specific embodiment of the present invention. In it, a copper wire lead is immersed in the bath, and then, with the help of a stamp, the excess solder covering the wire lead is removed.

As in the previous embodiment, in this case, a lead-free solder is used that does not contain other elements besides Sn, Cu and Ni. More precisely, a lead-free solder is used containing Cu at a concentration of 0.7 wt.%, Ni at a concentration of 0.05 wt.% And Sn, which accounts for the remaining percentage concentration. Lead-free solder melts at a temperature of 265 ° C in the bath. Under such conditions, the part, which is a copper wire output, is immersed in the bath. In order to increase ductility during broaching, the stamp and copper wire terminal are heated using a known heating device.

After drawing the copper wire lead coated with lead-free solder through the die in the same manner as described above, Cu along with the excess solder is removed from the copper wire lead. Then Cu is dissolved in the bath, resulting in a rapid increase in the concentration of Cu in the bath. There is also a rapid decrease in the concentration of Ni in the bath.

As in the case of using HASL for the subsequent soldering treatment, a lead-free replenishing solder containing 0.15 wt.% Ni and Sn was used, which accounts for the remaining percentage concentration. As described above, the soldering operation is repeatedly performed on a copper wire terminal. Replenishing lead-free solder is introduced into the bath as necessary and the Cu concentration and Ni concentration in the bath are measured. More precisely, the replenishing lead-free solder described above is added to the bath before the Cu concentration in the bath rises by a maximum of 0.5 wt.% Relative to its predetermined value, and the Ni concentration in the bath decreases by a maximum of 0.03 wt.% Relative to its predetermined quantities. The reference value is set in the same way as described in the previous embodiment, in which HASL is used in the process following soldering.

From the point of view of numerical evaluation, we obtained the same result of adjusting the concentration as in the process following the soldering using HASL. In another embodiment, the same result is obtained as in the soldering process using HASL. More precisely, when a lead-free solder containing Sn as the main component is introduced at any time, Cu at a concentration of more than 1.2 wt.% And Ni at a concentration of less than 0.01 wt.% Or more than 0.5 wt.%, Increasing the concentration Cu in the bath cannot be controlled, and concentration adjustment is not possible. When using a lead-free solder containing Cu in a concentration of less than 1.2 wt.% And Ni in a concentration ranging from 0.01 wt.% Inclusive to 0.5 wt.% Inclusive, it is possible to control the concentration. In particular, the regulation of the concentration is relatively quickly carried out using a lead-free solder in which the concentration of Cu is equal to or less than 0.7 wt.%. When the concentration of Cu is equal to or less than 0.5 wt.%, The regulation of the concentration is carried out even faster. The fastest and most reliable way to control the concentration of Cu is when a lead-free solder is added that does not contain Cu at all.

In another embodiment, in a lead-free solder containing Sn as the main component, Cu in a concentration equal to or less than 1.2 wt.%, Ni in a concentration ranging from 0.01 wt.% Inclusive to 0.5 wt.% Inclusive stabilize the concentration of Ni in the bath accurate to the appropriate interval, in particular from 0.02 wt.% inclusive to 0.08 wt.% inclusive. If the Ni concentration is in the range from 0.05 wt.% Inclusive to 0.3 wt.% Inclusive, the concentration of Ni in the bath is more quickly controlled with accuracy to the appropriate interval. The decreased Ni concentration in the bath is restored most quickly and the source of the formation of intermetallic compounds (Ni, Cu) ₆ Sn ₅ , which is formed as a result of an increase in the Ni concentration, is dissolved if the Ni concentration is in the range from 0.1 wt.% Inclusive to 0.2 wt. % inclusive.

Embodiments of the present invention are described by way of example only. The advantages of the present invention are provided by any of the above embodiments, even if the lead-free solder for refilling, in addition to Cu and Ni, contains germanium (Ge) and phosphorus (P) as an oxidation inhibitor at a concentration of about 0.1 wt.% Each. In any of the described embodiments, replenishing lead-free solder is added multiple times. The advantages of the present invention are provided even if the replenishing lead-free solder is added continuously, continuously or periodically in accordance with the soldering regime (for example, depending on the type of parts processed per day or the stability of the soldering process) and the Cu concentration and Ni concentration in the bath are measured. If the soldering operation occurs under stable conditions, the concentration is controlled by continuously, continuously or periodically adding replenishing lead-free solder and measuring the concentration of Cu and Ni. Using a known concentration meter in combination with a replenishment, automatic concentration control is performed and the concentration change is reduced.

Industrial applicability

Refillable lead-free solder and a method for controlling the concentration of Cu and Ni in an immersion brazing bath according to the present invention are highly effective production control means for use in a brazing operation in a particular process using HASL or a die in the process following the soldering.

Claims (5)

1. Lead-free solder for replenishing a tin-based lead-free solder bath containing copper and nickel, for immersion soldering of a part that is a printed circuit board coated with a copper film or a copper wire terminal, or a copper tape terminal, which is processed after soldering using an air knife or stamp, while replenishing lead-free solder contains Sn as the main component and at least Ni, the concentration of which is from 0.01 wt.% inclusive to 0.5 wt.% inclusive . 2. Lead-free solder according to claim 1, in which the concentration of Ni is from 0.05 wt.% Inclusive to 0.3 wt.% Inclusive. 3. Lead-free solder according to claim 1, in which the concentration of Ni is from 0.1 wt.% Inclusive to 0.2 wt.% Inclusive. 4. Lead-free solder according to any one of claims 1 to 3, which further comprises Cu, the concentration of which is up to 1.2 wt.%. 5. A method for controlling the concentration of copper and nickel in a tin-based lead-free solder bath containing copper and nickel for immersion soldering of a component consisting of a printed circuit board coated with a copper film or a copper wire terminal, or a copper tape terminal in which the component is immersed in the bath for soldering and return to the bath the solder that is removed from the part with an air knife or a stamp, while the concentration of copper and nickel in the solder is measured, and before the concentration of Cu in the solder increases by a maximum of 0.5 wt% rel relative to its predetermined value, and the Ni concentration will decrease by a maximum of 0.03 wt.% relative to its predetermined value, replenish the solder bath by introducing a lead-free solder containing Sn as the main component and at least Ni, the concentration of which is from 0.01 wt. wt.% inclusive up to 0.5 wt.% inclusive.

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